Method to increase electromigration resistance of copper using self-assembled organic thiolate monolayers

ABSTRACT

Methods and solutions for forming self assembled organic monolayers that are covalently bound to metal interfaces are presented along with a device containing a self assembled organic monolayer. Embodiments of the present invention utilize self assembled thiolate monolayers to prevent the electromigration and surface diffusion of copper atoms while minimizing the resistance of the interconnect lines. Self assembled thiolate monolayers are used to cap the copper interconnect lines and chemically hold the copper atoms at the top of the lines in place, thus preventing surface diffusion. The use of self assembled thiolate monolayers minimizes the resistance of copper interconnect lines because only a single monolayer of approximately 10 Å and 20 Å in thickness is used.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] Embodiments of the present invention relate to the field ofmaking reliable semiconductor devices, and in particular the preventionof the electromigration of copper lines.

[0003] 2. Discussion of Related Art

[0004] Advances in semiconductor manufacturing technology have led tothe development of integrated circuits having multiple levels ofinterconnect. In such an integrated circuit, patterned conductivematerial on one interconnect level is electrically insulated frompatterned conductive material on another interconnect level by films ofmaterial such as, for example, silicon dioxide. These conductivematerials are typically a metal or metal alloy. Connections between theconductive material at the various interconnect levels are made byforming openings in the insulating layers and providing an electricallyconductive structure such that the patterned conductive material fromdifferent interconnect levels are brought into electrical contact witheach other. These electrically conductive structures are often referredto as contacts or vias.

[0005] Other advances in semiconductor manufacturing technology havelead to the integration of millions of transistors, each capable ofswitching at high speed. A consequence of incorporating so many fastswitching transistors into an integrated circuit is an increase in powerconsumption during operation. One technique for increasing speed whilereducing power consumption is to replace the traditional aluminum andaluminum alloy interconnects found on integrated circuits with a metalsuch as copper, which offers lower electrical resistance. Those skilledin the electrical arts will appreciate that by reducing resistance,electrical signals may propagate more quickly through the interconnectpathways on an integrated circuit. Furthermore, because the resistanceof copper is significantly less than that of aluminum, thecross-sectional area of a copper interconnect line, as compared to analuminum interconnect line, may be made smaller without incurringincreased signal propagation delays based on the resistance of theinterconnect.

[0006] As device dimensions shrink, so does conductor width—leading tohigher resistance and current density. Increasing current density leadsto the phenomenon of electromigration. Electromigration is generally themovement of atoms in a metal interconnect in the direction of currentflow. Most metal atoms that move during electromigration are displacedat the top of an interconnect line where there is no barrier layer toprevent their displacement. This is called surface diffusion. Surfacediffusion can cause vacancies, which lead to voids and hillocks, andultimately to electromigration failure of the device.

[0007] Others have tried to solve this problem by alloying the copperlines with another metal. One method includes the doping of the entiremetal interconnect line with metallic dopants in order to preventmovement of the atoms of the metal interconnect line in the direction ofthe current flow. The dopants will either physically inhibit themovement of copper atoms or enlarge the copper grain size such that thediffusion path of the copper atoms is eliminated. However, blanketdoping of the metal interconnect layer results in an increasedresistivity of the interconnect layer, which degrades performance of thesemiconductor device. In response to this increased resistivity theportion of the copper line that is doped has been decreased to only theouter edges or the top of the line to prevent surface diffusion. Shuntlayers have also been used to prevent electromigration. Shunt layers arethin electrically conductive layers formed around the copper lines.Shunt layers prevent electromigration by physically inhibiting themovement of copper atoms. Additionally, shunt layers are several hundredangstroms thick and result in increased line to line leakage due tonon-selective deposition. But, due to the further scaling down ofdevices and the narrowing of copper interconnect lines, the resistancecaused by the doping of the outer layers of the lines and by the shuntlayers has become significant.

[0008] Embodiments of the invention provide processes and devices thatmore effectively reduce electromigration, in particular surfacediffusion, without significantly increasing conductor resistance. Theseembodiments are valuable in minimizing the electromigration of scaleddown copper lines.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1a is an illustration of a cross-sectional view of a dualdamascene structure after the dielectric layer has been etched to formboth vias and trenches.

[0010]FIG. 1b is an illustration of a cross-sectional view of a dualdamascene structure after the vias and trenches have been filled with acopper layer.

[0011]FIG. 1c is an illustration of a cross-sectional view of a dualdamascene structure after the copper layer has been polished.

[0012]FIG. 2 is an illustration of a cross-sectional view of the copperdamascene structure of FIG. 1 after a self-assembled organic monolayerhas been covalently bound to the copper interfaces.

[0013]FIG. 3 is an illustration of a thiolate molecule reacting with ametal surface.

[0014]FIG. 4 is an illustration of a cross-sectional view of a copperinterface on which a self-assembled thiolate monolayer has been formed.The chemical bond between the copper and sulfur atoms is featured.

[0015]FIG. 5 is an illustration of a cross-sectional view of the copperdamascene structure of FIG. 1c after a self-assembled organic monolayerhas been formed on the copper interfaces and a silicon based layer hasbeen formed over the thiolate monolayer.

[0016]FIG. 6 is an illustration of a cross-sectional view of a copperinterface on which a monolayer of 11-trichlorosilyl undecyl thioacetatehas been formed, one which a silicon based layer has been formed. Thechemical bonds between the copper interface, the monolayer, and thesilicon based layer are featured.

[0017]FIG. 7 is a flow chart showing a copper damascene process offorming a semiconductor device including forming an organic layer thatis covalently bound to a metal layer.

[0018]FIG. 8 is a flow chart showing a copper damascene process offorming a semiconductor device including a polishing step during whichan organic layer that is covalently bound to a metal layer is formed bya slurry.

[0019]FIG. 9 is a flow chart showing a copper damascene process offorming a semiconductor device including a cleaning step during which anorganic layer that is covalently bound to a metal layer is formed by arinse.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

[0020] Devices and methods employing thiolate layers to prevent theelectromigration of copper interconnects are described. In the followingdescription numerous specific details are set forth to provide anunderstanding of the embodiments of the present invention. It will beapparent, however, to those skilled in the art and having the benefit ofthis disclosure, that the embodiments of the present invention may bepracticed with materials and processes that vary from those specifiedhere.

[0021] Terminology

[0022] The terms chip, integrated circuit, monolithic device,semiconductor device or component, microelectronic device or component,and similar terms and expressions, are often used interchangeably inthis field. The present invention is applicable to all the above as theyare generally understood in the field.

[0023] The terms metal line, trace, wire, conductor, signal path andsignaling medium are all related. The related terms listed above, aregenerally interchangeable, and appear in order from specific to general.In this field, metal lines are sometimes referred to as traces, wires,lines, interconnects or simply metal.

[0024] The terms contact and via both refer to structures for electricalconnection of conductors from different interconnect levels. These termsare sometimes used in the art to describe both an opening in aninsulator in which the structure will be completed, and the completedstructure itself. For purposes of this disclosure contact and via referto the completed structure.

[0025] The term copper interface refers to the copper surface that isexposed after a copper layer has been planarized by chemical mechanicalpolishing. A copper interface is typically an exposed copper line or viathat will be subsequently covered with another layer to form afunctional semiconductor device.

[0026] The term self-assembled monolayer refers to a film that is formedby molecules that will react with a surface in such a way that they lineup in a uniform manner to create a homogeneous film that is only onemolecule thick. Specifically, they “self assemble” because eachself-assembling molecule forms a highly selective bond with copper andorientates itself perpendicular to the face of the copper surface.Through this reaction a uniform monolayer film is formed.

[0027] The terms thiol, thiolate, and X-alkanethiolate all refer tocompounds containing sulfur. A thiol is a sulfur containing compoundwhere the sulfur atom is terminated by hydrogen (X—S—H). A thiolate is amore general term, referring to compounds where the sulfur is bound toany substituent including copper (X—S—Y). X-alkanethiolates refer tothiolates where the sulfur is bound to an organic compound that isalkane based and the alkane is terminated by a substituent X(X—(CH₂)_(n)—S—Y).

[0028] Embodiments of the Invention

[0029] Methods and solutions for forming organic layers covalently boundto metal layers are presented along with devices containing organiclayers covalently bound to metal layers. In a preferred embodiment,organic monolayers that form covalent bonds to metal by self-assemblyare utilized to prevent the electromigration and surface scattering ofcopper atoms while minimizing the resistance of the interconnect lines.Electromigration and surface diffusion is prevented because the organiclayer is covalently bound to the metal atoms in the metal interface. Thecovalent bonds will chemically hold the metal atoms in place.Additionally, in a preferred embodiment, the organic molecules in theorganic layer are relatively large and will help hold the metal atoms inplace because it is virtually impossible for metal atoms to migrate whencovalently bound to large organic molecules. The resistance of theinterconnect lines is minimized because, in a preferred embodiment, onlya single monolayer of organic material is used.

[0030] In a preferred embodiment the organic layer is a self assembledthiolate monolayer and the metal layer is copper. Self assembledthiolate monolayers are valuable because they can form thin (10 Å to 20Å) layers that will cap copper interconnect lines and chemically holdthe copper atoms at the copper interfaces at the top of the lines inplace, thus preventing electromigration and surface scattering.

[0031] Copper interconnect lines are formed by way of a damascene, orinlaid, metal process. Typically a dual damascene process is used toform both vias and trenches in a single layer. FIG. 1a illustrates adual damascene structure 100 after vias 110 and trenches 120 havealready been etched into dielectric layer 130. A barrier layer 140 canoptionally be formed over the patterned dielectric layer 130. FIG. 1billustrates the dual damascene structure after the vias 110 and trenches120 have been filled with copper 150. The excess copper layer 160 isthen polished using chemical mechanical polishing (CMP), resulting inthe planarized dual damascene structure illustrated in FIG. 1c. AfterCMP, copper interfaces 170 are exposed at the top of the copper lines.

[0032]FIG. 2 illustrates an embodiment of the present invention where anorganic layer 210 is has formed covalent bonds with the copper interface170 by self assembly to form a monolayer. In a preferred embodiment theorganic molecules are thiolates. Thiolates are sulfur containingmolecules (X—S—Y) that react with metallic interfaces and form linkagesbetween the sulfur atom and the metal surface (X—S-M). The species Y ina thiolate (X—S—Y) is a surface active agent that reacts with the copperinterface to form the bond between the sulfur of the thiolate and thecopper of the interface. The species Y can be any surface active agentincluding but not limited to H, COOH, CH₃, Cl, and F, where using Cl orF will aid in the solubility of the thiolate molecule. The reactionbetween the species Y of a thiolate and a metal interface to form anX—S-M linkage is illustrated in FIG. 3. As the thiolate molecule 310comes into close proximity with the metal surface 320 (illustrated at 3a) the sulfur atom 330 will become attracted to a metal atom 340 at themetal surface and begin to form a bond 350 with the metal atom 340, asillustrated at 3 b. Next, as illustrated at 3 c, a covalent bond 360forms between the sulfur atom 330 and the metal atom 340, breaking thebond between the sulfur atom 330 and the species Y 370 to form an X—S-Mlinkage.

[0033] In an embodiment of the present invention, as illustrated in FIG.4, this reaction will occur between X-alkanethiolate molecules 410 andthe exposed copper atoms 420 at a copper interface to form aself-assembled thiolate monolayer 430. Covalent bonds are the strongesttype of bond. This chemical bond will chemically hold the copper atomsin place to prevent surface diffusion. This is in contrast to copperalloys and metal shunt layers that merely block the movement of copperatoms because they do not form covalent bonds with copper. Physicallyblocking the copper atoms is not as effective as the chemical bondformed by thiolates with copper. The species X may be any type ofsubstituent, but is preferably an organic substituent. Organicsubstituents will not cause shorts or increased leakage for narrow linesbecause they are nonconductive. Additionally the species X can bereadily manipulated. The species X can be selected to tailor thediffusion coefficient of copper at the copper interface. The larger themolecule that is attached to the copper atoms, the less the copper atomswill be able to move. The molecular species X can be modified to be aslarge or bulky as possible. It is virtually impossible for a copper atomto diffuse at room temperature when it is attached to a sizeable organicgroup because of reduced mobility. In a preferred embodiment, thesesizeable organic substituents will be in the form of alkanes that arepart of the thiolates. These types of molecules will be referred to asX-alkanethiolates. The optimal size of the X-alkanethiolates(X—(CH₂)_(n)—S—Y) is where n=10 or 11. But n may be any number of alkane(CH₂) groups. An X-alkanethiolate where n=10 is called an undecylthiolate and an X-alkanethiolate where n=11 is called an octadecylthiolate.

[0034] An additional advantage of X-alkanethiolates, and in particularundecyl and octadecyl thiolates, is that they improve the corrosionresistance of copper films to oxidation. X-alkanethiolates areanti-corrosive because they form densely packed monolayer structures andform strong covalent bonds with the copper atoms that prevent theoxidation of Cu. The anti-corrosive properties are increased when ahydrophobic (water repelling) substituent is chosen as the X-group forX-alkanethiolates.

[0035] Additionally, undecyl and octadecyl thiolates will form amonolayer having the optimal thickness. The thickness of the monolayeris preferably between 10 Å and 40 Å and optimally between 10 Å and 20 Å.It is valuable that the monolayer has a thickness greater than 10 Å toprotect against corrosion. Also, it is valuable that the monolayer has athickness less than 40 Å to minimize the resistance of the interconnectline.

[0036] The X-group substituent of X-alkane thiolates can also be chosento promote adhesion between different surfaces, and in particularbetween copper interfaces and materials formed on the copper interfaces.The type of substituent that will work the best depends on the type ofmaterial that will be deposited on the copper interface. Typically asilicon based material is used to form an interlayer dielectric (ILD) oretch stop (ES) layer on the copper interface. Silanes are a good choicefor the X-group of X-alkanethiolates when silicon based materials areformed on the copper interface. FIG. 5 shows a covalently bound organicmonolayer 510 acting as an adhesive between copper interfaces 170 of acopper damascene structure 500 and a silicon based layer 520.

[0037] An ideal adhesion promoting thiolate is 11-trichlorosilyl undecylthioacetate. This molecule has been shown to promote adhesion betweensilicon dioxide and metals such as gold. FIG. 6 illustrates a monolayerof 11-trichlorosilyl undecyl thioacetate 610 formed on a copperinterface 170. An S-Cu bond is formed between the copper atoms 640 andthe thiolate 610. And, an Si—O—Si bond 620 is formed between an ILD oran ES (etch stop) layer 630 and the thiolate 610. Typically the siliconbased layer 630 is an etch stop layer made of SiN or SiON. The Si—O—Sichemical bond is the key to the strong adhesion between the copperinterface and the silicon based layer.

[0038] The organic layer covalently bound to a metal layer can be formedby several different methods. Embodiments of some exemplary methods arepresented below. In general, an organic layer covalently bound to ametal layer is formed by applying a solution containing self-assemblingorganic molecules to a metal interface. The self-assembling organicmolecules will adsorb from the solution onto the metal interface to formthe organic layer. In a preferred embodiment the organic layer is amonolayer, that is, it has a thickness of one organic molecule. To forma monolayer, the concentration of self-assembling molecules in thesolution is such that there is one self-assembling molecule for everyone metal atom to which the solution will be exposed. By using thisconcentration it will be ensured that only a monolayer of theself-assembling molecules is formed. The amount of non-conductiveorganic material used is thereby minimized, that will in turn minimizethe increase in resistance of the metal lines. Additionally, themonolayer is very thin and can be etched away before forming a via. In apreferred embodiment the self-assembling organic monolayer is athiolate.

[0039] In an embodiment, the organic layer covalently bound to a metallayer is formed after CMP (chemical mechanical polishing). FIG. 7depicts a flow diagram of a method embodying a typical damascene processemploying an embodiment of the present invention. At block 710 apatterned dielectric layer is formed. Typically the dielectric layerwill be patterned to have several trenches and vias using the dualdamascene process described above. At block 720 a metal layer is formedover the patterned dielectric layer. Then, at block 730 the metal layeris polished. After polishing, the covalently bound organic layer isformed on the polished metal interfaces at block 740 using a solutioncontaining self-assembling organic molecules. In a preferred embodimentthe self-assembling organic molecules are thiolates. The solution can besprayed or poured onto the substrate or the substrate may be immersedinto a bath containing the solution. In an embodiment, the solutioncontaining the self-assembling organic molecules is a mixture of theself-assembling organic molecules and a solvent. The solvent is chosenbased on the specific self-assembling molecule that needs to besolvated. In an embodiment, solvents such as isooctane, chloroform,tetrahydrofuran, acetonitrile, acetone, and ethanol may be used. In analternate embodiment a gaseous solution of the self-assembling organicmolecules is used. The gaseous solution would contain a gaseous form ofthe self-assembling organic molecules. In a preferred embodiment theorganic molecules are gaseous thiolates. A carrying gas such as argonmay also be mixed with the gaseous organic molecules. The gas would beapplied to the substrate by spraying or by exposing the substrate to thegas. Optionally, an anneal is subsequently performed at block 750 on theorganic layer to better align the self-assembled organic molecules intoa uniform structure.

[0040] In an alternate embodiment, the organic layer is formed duringthe chemical mechanical polishing (CMP) step. This has the advantage ofnot adding an extra step to the processing of a semiconductor device.FIG. 8 depicts a flow diagram of a method where the covalently boundorganic layer is formed during the CMP step. At block 810 a patterneddielectric layer is formed. Next, at block 820 a copper layer is formedover the patterned dielectric layer. The CMP step is then performed atblock 830. During this CMP step an organic layer that will form covalentbonds with the metal layer is formed on the metal layer by using aslurry containing self-assembling organic molecules. An exemplary slurrywould contain self-assembling organic molecules, an abrasive, anoxidizing agent, and a chelating buffer system. The concentration ofself-assembling organic molecules in the slurry is such that aself-assembled organic monolayer is formed. In a preferred embodimentthe self-assembling organic molecules are thiolates. Typical abrasivesinclude silica, alumina, and ceria. The oxidizing agent is typicallyhydrogen peroxide. And, the chelating buffer system is typically amixture of citric acid and potassium citrate.

[0041] In an alternate embodiment, the covalently bound organic layer isformed during the cleaning performed after CMP. This method has theadvantage of not adding an extra step to the processing of asemiconductor device. FIG. 9 depicts a flow diagram of a method wherethe covalently bound organic layer is formed during cleaning. At block910 a patterned dielectric layer is formed. Next, at block 920 a metallayer is formed over the barrier layer. The metal is then polished atblock 930. Following the polishing, the copper layer is cleaned at block940 with a rinse containing self-assembling organic molecules that willform covalent bonds with the metal interface. During the cleaning stepthe covalently bound organic layer is formed. An exemplary rinse wouldcontain self-assembling organic molecules, water, and an alcohol such asisopropyl alcohol or a weak acid such as citric acid. In a preferredembodiment the self-assembling organic molecules are thiolates. Theconcentration of self-assembling organic molecules in the rinse is suchthat a self-assembled organic monolayer is formed.

[0042] The components of the solutions, slurries, and rinses presentedabove may remain in their original chemical form as they existed beforethey were added to the mixture or they may combine to form chemicalcompounds or ionic species different from the original components asthey existed before they were added to the mixture.

CONCLUSION

[0043] Embodiments of the present invention provide methods andsolutions for forming a semiconductor device containing an organic layerthat is self-assembled and covalently bound to a metal interface.Various embodiments of such a device are also presented. In a preferredembodiment the covalently bound organic layer is a thiolate and themetal to which it is covalently bound is copper. These embodimentsemploying covalently bound organic layers prevent the electromigrationand surface diffusion of metal lines, and in particular of copper lines.Embodiments of the present invention are valuable in minimizing theelectromigration of scaled down copper lines without significantlyincreasing the resistance of copper lines.

[0044] Other modifications from the specifically described devices,solutions, and processes will be apparent to those skilled in the artand having the benefit of this disclosure. Accordingly, it is intendedthat all such modifications and alterations be considered as within thespirit and scope of the invention as defined by the subjoined claims.

1. A method of forming a semiconductor device comprising: forming adielectric layer; forming a copper layer over the dielectric layer;forming an adhesion layer of 11-trichlorosilyl undecyl thioacetate onthe copper layer, wherein the adhesion layer is covalently bound to thecopper layer; and forming a silicon dioxide layer on the adhesion layer11-trichlorosilyl undecyl thioacetate, wherein the adhesion layer iscovalently bound to the silicon dioxide layer.
 2. (cancelled)
 3. Themethod of claim 1 wherein the adhesion layer is a monolayer.
 4. Themethod of claim 3 wherein the monolayer is formed by exposing the copperlayer to a solution having a concentration of one organic molecule thatwill form a covalent bond with a copper atom for every one copper atomof the copper layer.
 5. The method of claim 1 wherein the adhesion layeris formed with 11-trichlorosilyl undecyl thioacetate.
 6. The method ofclaim 5 wherein the adhesion layer is formed by exposing the copperlayer to a chemical mechanical polishing slurry containing11-trichlorosilyl undecyl thioacetate.
 7. (cancelled)
 8. The method ofclaim 5 wherein the adhesion layer is formed by exposing the copperlayer to gaseous self-assembling 11-trichlorosilyl undecyl thioacetate.9. The method of claim 5 wherein the adhesion layer is formed byexposing the copper layer to a solution containing self-assembling11-trichlorosilyl undecyl thioacetate and a solvent.
 10. The method ofclaim 9 wherein the solvent is selected from the group consisting ofisooctane, chloroform, tetrahydrofuran, acetonitrile, acetone, orethanol.
 11. A method of forming a semiconductor device comprising:forming a patterned dielectric layer; forming a copper layer over thepatterned dielectric layer; polishing the copper layer by chemicalmechanical polishing (CMP) to expose at least one copper interface; and,applying a post-CMP rinse comprising a thiolate, an alcohol, a weakacid, and water to form a thiolate layer on the at least one copperinterface.
 12. The method of claim 11 further comprising forming abarrier layer over the dielectric layer.
 13. The method of claim 11further comprising annealing the thiolate layer.
 14. The method of claim11 wherein the thiolate molecules are 11-trichlorosilyundecylthioacetate.
 15. The method of claim 11 wherein the thiolate moleculesare octadecanethiol. 16-30 (cancelled)
 31. A method of forming asemiconductor device, comprising: forming a patterned dielectric layer;forming a copper layer over the patterned dielectric layer; polishingthe copper layer with a chemical mechanical polishing (CMP) slurry toexpose at least one copper interface, the CMP slurry comprising anX-alkanethiolate, an abrasive, an oxidizing agent, and a chelatingbuffer system, wherein the CMP slurry forms a covalently bound layer ofthe X-alkanethiolate on the copper layer.
 32. The method of claim 31,wherein polishing the copper layer with the CMP slurry comprisespolishing the copper layer with the CMP slurry containing theX-alkanethiolate comprising undecylthiolate.
 33. The method of claim 31,wherein polishing the copper layer with the CMP slurry comprisespolishing the copper layer with the CMP slurry containing theX-alkanethiolate comprising octadecylthiolate.